This Customer Support Ruling discusses the use of postage stamps
with advertising stickers in an advertising campaign.

An inquiry was made as to whether it
is possible to obtain a permit to conduct an advertising campaign
involving United States postage stamps. In the course of this
campaign, postage stamps would be attached to advertising stickers
and resold to individuals who would use the combined stickers and
postage stamps to mail their personal and business letters.

The Postal Service does not issue
permits for the use of decorative seals or stickers on mail matter.
Indeed, the Postal Service does not encourage the use of decorative
seals or stickers on the address side of mail; and mail acceptance
personnel may from time to time reject envelopes bearing stickers
which are located where they could impair mail processing functions,
or which imitate postage stamps by having such characteristics as
words, numerals, or markings which might indicate a postage value or
a postal service.

The Criminal Division of the United
States Department of Justice has advised that the use of postage
stamps in the manner referred to above would constitute a violation
of 18 U.S.C. 475. That statue provides, in pertinent part, that
"Whoever...attaches to any...obligation ...of the United States any
business or professional card, notice, or advertisement, or any
notice or advertisement, whatever, shall be fined not more than
$500". The Justice Department has further advised that it would
not, as a general proposition, be disposed to forego prosecution of
those who violate section 475. Inquiries regarding the enforcement
of section 475 and other provisions contained in chapter 25 of title
18 should be directed to the Office of Chief Counsel, United States
Secret Service, Department of the Treasury, 1800 G Street NW,
Washington, DC 20223.